Polyethylene has desirable properties that have helped to make it the highest volume polymer manufactured. Ziegler-Natta catalysts are a mainstay for polyolefin manufacture and are used in slurry, solution, and gas-phase polymerizations. Certain applications, such as blow-moulding, require a careful balance of rheological properties, and there is a continuing effort to develop polyethylene with improved properties.
A property that is especially important for blow-molding applications is the extrudate swell. When a resin exhibits too little swell, it can become difficult or impossible to properly fill out the extremities of the mold, such as the handle of a blow-molded bottle.
Until recently, predicting extrudate swell of a particular resin was not a simple feat. In particular, identifying the relative effects of average mass and polydispersity on swell proved difficult, as shown by Doelder et al Journal of Non-Newtonian Fluid Mechanics 152 (2008) 195-202.
US 2008/0132640 predicts a high swell and high melt-strength using a Ziegler-Natta catalyst in two slurry loop reactors in series, whereby the ratio of the weight average molecular weight Mw of the high molecular weight fraction to Mw of the low molecular weight fraction is 8:1. However, the disadvantages of this process/produced polyethylene are that the Charpy impact resistance needs still to be improved for blow-moulding resins, in particular for applications such as tight-head drums.
Another possible solution is to crosslink the polyethylene in order to increase extrudate swell. For example, U.S. Pat. No. 5,486,575 improves the properties of a polyethylene resin prepared from a chromium catalyst by using an organic peroxide. U.S. Pat. Nos. 4,390,666 and 4,603,173 use peroxides to crosslink a polyethylene blend containing high and low molecular weight components. U.S. Pat. No. 6,706,822 discloses using peroxides with polyethylene having a broad molecular weight distribution to improve melt swell. U.S. Pat. No. 5,486,575 discloses using peroxides with polyethylene prepared with chromium catalysts. While some properties can be improved by crosslinking with peroxides, there are issues with this approach. The radicals produced can interact deleteriously with other additives. Furthermore, it is difficult to predict the effect of crosslinking on rheological properties. Reported results vary significantly from resin to resin, even when the resins are produced using similar catalyst technologies. Peroxides add an extra component to the composition, and they require careful handling and storage, which adds to the overall cost. It would be desirable to improve properties of a polyethylene resin for use in blow-moulding applications without using peroxides, since these generally induce lower impact properties.
Another approach to improve the rheological and mechanical properties of a polyethylene resin, disclosed in U.S. Pat. Nos. 4,336,352, 5,422,400, and U.S. Pat. No. 6,743,863, is to use blends containing three resin components. The blends can be made in a three-step polymerisation process. U.S. Pat. No. 4,336,352 states that mixtures of high and low molecular weight polyethylene (or blends produced by a multi-step polymerization process) have such a low die swell that blow-molded bottles of consistent quality are difficult to obtain. U.S. Pat. No. 5,422,400 states that earlier approaches with two-component mixtures have important limitations such as the need to use an ultrahigh molecular weight polyethylene having a minimum intrinsic viscosity. They overcome these limitations by using a three-step polymerization. Unfortunately, a three-component blend or a three-step polymerization is complicated and requires additional equipment. Polyethylene produced in two-step or even one-step polymerizations in the presence of Ziegler-Natta catalysts are generally known. For example, U.S. Pat. No. 6,878,454 teaches films prepared from bimodal polyethylene prepared using a variety of one stage and multistage processes. However, the polyethylene has a greater than 50% by weight fraction of the high molecular weight component.
Despite continued efforts to improve polyethylene properties for blow molding, there is a need for a process that can produce polyethylene with a high extrudate swell, but which does not require the extra equipment and complications of a three-step polymerization process. There is also a need to increase the extrudate swell and the charpy impact resistance of the polyethylene resin for certain blow-moulding applications, such as to produce tight-head drums. Finally, there is also a need to increase the extrudate swell, the charpy impact resistance and the ESCR for certain blow-moulding applications, such as to produce fuel tanks.